Recent years have seen further enhancement in functionality of electronic devices such as mobile information devices and information appliances following the development of digital technology. With the enhanced functionality in these electronic devices, miniaturization and increase in speed of semiconductor elements for use therein are rapidly advancing. Among these, application of large-capacity nonvolatile memories represented by a flash memory is rapidly expanding. In addition, as a next-generation new-type nonvolatile memory to replace the flash memory, research and development on a variable resistance semiconductor memory device which uses what is called a variable resistance element is advancing. Here, variable resistance element refers to an element having a property in which resistance reversibly changes according to electrical signals, and capable of storing information corresponding to the value of the resistance value in a nonvolatile manner.
As an example of such a variable resistance element, there is proposed a nonvolatile memory device having a variable resistance layer in which transition metal oxides having different oxygen content percentages are stacked. For example, PTL 1 discloses selectively causing oxidation reaction and reduction reaction in an interface where an electrode and a variable resistance layer having a high oxygen content percentage are in contact, to stabilize resistance change.
The conventional variable resistance element includes a lower electrode, a variable resistance layer, and an upper electrode. The variable resistance layer is of a stacked structure including a first variable resistance layer and a second variable resistance layer, and the first and second variable resistance layers include the same type of transition metal oxide. The oxygen content percentage of the transition metal oxide in the second variable resistance layer is higher than the oxygen content percentage of the transition metal oxide in the first variable resistance layer. With such a structure, when voltage is applied to the variable resistance element, most of the voltage is applied to the second variable resistance layer which having a higher oxygen content percentage and exhibits a higher resistance value. Furthermore, oxygen, which can contribute to the reaction, is abundant in the vicinity of the interface. Therefore, oxidation reaction and reduction reaction occur selectively at the interface between the upper electrode and the second variable resistance layer, and resistance thereby changes stably.